Piston Kit OS 1.00 - Yamaha YFM250 / YFB250
Piston Kit OS 1.00 - Yamaha YFM250 / YFB250 is backordered and will ship as soon as it is back in stock.
Delivery and Shipping info
Delivery and Shipping info
- Standard delivery (2-4 working days) - £4.99 on eligible UK mainland orders under £50.
- Express delivery - £6.99 on eligible UK mainland orders placed before 2pm.
- Large equipment including international orders are subject to different delivery rates and timescales.
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Piston Kit OS 1.00 - Yamaha YFM250 / YFB250 is backordered and will ship as soon as it is back in stock.
Restore peak compression and dependable power to Yamaha YFM/YFB 250 ATVs with the OS 1.00 Piston Kit. This oversize bore kit is engineered for Timberwolf, Bear Tracker, and Bruin models, covering 1989 - 2001 and 2005 - 2006, delivering a +1.00 mm bore to compensate wear and reclaim factory-like response. The kit arrives as a complete top-end refresh package, designed for a straightforward install so you can hit the trail or farm hard without lengthy downtime.
Over time, piston-ring wear, bore taper, and ring-land scuffing sap compression and throttle response, especially under heavy trail work or plowing. The OS 1.00 piston kit targets these wear points with a forged aluminum piston and a full ring-set, wrist pin, and circlips to restore compression, heat tolerance, and consistent power. Built as an all-in-one aftermarket replacement, it delivers reliable, repeatable performance in demanding Yamaha YFM/YFB 250 applications.
What's Included
All essential components for a complete top-end refresh are included.
- Piston
- Rings
- Wrist Pin
- Circlips
Related gaskets, seals, and top-end hardware are not included and are typically purchased separately.
OEM & Aftermarket Cross-Reference Information
| Part Number | Brand / Type | Cross-Reference Notes |
|---|---|---|
| 4BD-11631-00-Y0 | Yamaha OEM (Reference) | Original equipment reference for YFM/YFB 250 engines; used for cross-checking aftermarket oversize kits |
| OS 1.00 Piston Kit | Aftermarket Replacement | Oversize +1.00 mm kit designed to replace the OEM reference; direct-fit alternative |
Fitment
| Manufacturer | Models / Series | Years / Details |
|---|---|---|
| Yamaha | YFM 250 Bear Tracker | 1989 - 2001 |
| Yamaha | YFB 250 Timberwolf | 1989 - 2001 |
| Yamaha | YFM 250 Bruin | 2005 - 2006 |
Specifications
| Attribute | Details |
|---|---|
| OEM Reference Part Numbers | 4BD-11631-00-Y0 |
| Type | Piston Kit OS 1.00 (Oversized) |
| Manufacturer | Aftermarket Replacement |
| Application | Yamaha YFM/YFB 250 engines (Timberwolf, Bear Tracker, Bruin) - 1989 - 2001 and 2005 - 2006 |
| Piston Material | Forged aluminum |
| Bore Size (Oversize) | +1.00 mm (72.00 mm) |
| Includes | Piston, rings, wrist pin, and circlips |
Installation Notes
- Inspect the cylinder bore for scoring and taper; verify bore size is within oversize kit specifications before installation.
- Use a proper ring compressor to install the piston; ensure rings are oriented correctly and gaps are staggered per OEM guidelines.
- Lubricate the piston, rings, and wrist pin with assembly lube before final assembly to prevent dry-start wear.
- Install the wrist pin and circlips carefully; confirm clips are fully seated and not projecting into the crankcase.
- Clean all gasket surfaces and ensure the cylinder head is clear of debris; replace gaskets if reused during top-end service.
- Follow a cautious break-in procedure: throttle lightly for the first 20 - 30 minutes, gradually increasing RPM to seat rings and verify compression before sustained high-rpm operation.
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